Feedback in audio esp wrt op-amps.

[Arny Krueger]

"William Sommerwerck" <grizzledgeezer@comcast.net> wrote in message
news:qtGdndAVmYMhW1TbnZ2dnUVZ_hisnZ2d@comcast.com...

And somehow, magically, the golden-ear boys (is's almost always
boys) think that it matters that what they do to the signal that
comes *off* the CD makes so much difference that they can hear
the difference in the oxygen content of the interconnect wiring,
or 0.06 percent distortion when the producer added 30% of his
own, because he liked the effect.

What you say is intellectually logical, but it seems that post-recording
distortions can be plainly audible, regardless of the quality of the
recording.

30 years of experience with bias-controlled listening tests says that
*seems* and *is* can be two different things. Intellectual logic has this
interesting tendency to rule, once the comparison is based on just
listening.

When I reviewed, I made final judgements with my own live, undoctored
recordings.

The fun begins when you level-match, time-synch and eliminate other
non-audible cues.

 

[Arny Krueger]

"William Sommerwerck" <grizzledgeezer@comcast.net> wrote in message
news:O4SdneIVPJnlWlTbnZ2dnUVZ_gSdnZ2d@comcast.com...

There was part of a thread a while back about how adding negative
feedback can create higher order harmonic distortion products than
exist open-loop in an amplifier stage.

This premise is NOT correct. Do not believe everything you read on
the Internet.

Feedback done correctly ADDS nothing. Perhaps what you are thinking
about is that feedback is generally more effective at reducing low-order
distortion compared to reducing high order distortion. Feedback
(implemented correctly) does not INCREASE either form of distortion.
It reduces them both.

I'm sorry, Mark, but this has been known for decades, and was not
established by audiophile reviewers -- the reduction of the overall
distortion level is accompanied by an increase in higher-order harmonics.

I apologize for not having a reference.

Something to do with reality intruding on fantasy. Normally, a reduction of
the overall
distortion level is accompanied by a similar but possibly smaller decrease
in higher-order harmonics. A decrease in all forms of distortion is the
primary effect. The shift towards larger percentages (but not larger
amounts) of higher order distortion is a secondary effect.

One possible exception was described by Don - relating to marginal
stability.

Another common situation is where the open-loop gain of the amplifier inside
the loop simply falls with increasing frequency. Very common, particularly
with op amps. The higher harmonics are still reduced, but they may be
reduced by a smaller amount than the lower harmonics. This leads to the
higher harmonics being a bigger slice of a significantly smaller pie. The
smaller pie is the stronger effect, so the size of all harmonics is still
reduced.

When the pie is as sour-tasting as nonlinear distortion is in reproduction
equipment, I'm always in favor of significantly smaller pies!

The source of this myth is the mistaken idea that negative feedback
regenerates the audio signal, and the nonlinearity of the amplifier leads to
higher order products of the regenerated harmonics and the nonlinearity of
the amp. This ignores the fact that the regenerated signal is brought back
in out-of-phase, and has the primary result of reducing the high order
harmonics.

 

[Arny Krueger]

"Kevin Aylward" <none@none.com> wrote in message
news:RYjyi.19820$mo.8773@newsfe4-win.ntli.net...

Well, it is trivially obvious that a pure square law device, with a
*small* amount of feedback will generate 3rd harmonic distortion, that was
never orginally there, from the mixing of the second and the fundamental.
It is also true that for such low levels of feedback, although the total
thd is less, the new 3rd component may sound more objectionable to those
goldern ears. However, assuming *sufficient* feedback is applied, the
final distortion will be audiable less noticable.

Let's put this into a real-world perspective. The LM 4562 has a typical GBW
of 55 MHz. If it is a typical compensated op amp, that means that its gain
at 1 KHz is 55,000. The feedback factor at 60 dB gain (x1,000) and 1 KHz is
thus 55. Based on its specs, its open-loop nonlinear distortion at 1 KHz is
55,000 times its unity gain distortion @ 1KHz is less than 2%. At 60 dB
gain, negative feedback drops this to about 0.035 %. Let's assume the worst
case - the nonlinear distortion is all second harmonic. Then, when fed back,
0.035% or less of the 0.035% second order distortion undergoes conversion to
third harmonic.

IOW with feedback, there is now 0.00001225% or less third harmonic in
addition to the 0.035% second. I wouldn't expect anybody to hear the 0.035%
second order nonlinear distortion, and definitely not the 0.00001225% third.

 

[Eeyore]

Mark wrote:

Eeyore wrote:
There was part of a thread a while back about how adding negative feedback can
create higher order harmonic distortion products than exist open-loop in an
amplifier stage.

This premise is NOT correct. Do not believe everything you read on
the Internet.

Feedback done correctly ADDS nothing. Perhaps what you are thinking
about is that feedback is generally more effective at reducing low
order distortion compared to reducing high order distortion. Feedback
(implemented correctly) does not INCREASE either form of distortion.
It reduces them both.

I know it decreases overall THD numbers. I'm not one of those nuts who's anti-NFB
per se.

What is the case AIUI is that NFB can create 'new' (higher) harmonics that don't
exist with the open-loop situation. It's down to the maths of how feedback works.

Graham

 

[Eeyore]

William Sommerwerck wrote:

And somehow, magically, the golden-ear boys (is's almost always
boys) think that it matters that what they do to the signal that
comes *off* the CD makes so much difference that they can hear
the difference in the oxygen content of the interconnect wiring,
or 0.06 percent distortion when the producer added 30% of his
own, because he liked the effect.

What you say is intellectually logical, but it seems that post-recording
distortions can be plainly audible, regardless of the quality of the
recording.

Absolutely true.

The idea that you can 'get away' with sloppy circuitry for replay because the
source was in some way 'impaired' is totally false.

Graham

 

[Eeyore]

Scott Dorsey wrote:

William Sommerwerck <grizzledgeezer@comcast.net> wrote:
And somehow, magically, the golden-ear boys (is's almost always
boys) think that it matters that what they do to the signal that
comes *off* the CD makes so much difference that they can hear
the difference in the oxygen content of the interconnect wiring,
or 0.06 percent distortion when the producer added 30% of his
own, because he liked the effect.

What you say is intellectually logical, but it seems that post-recording
distortions can be plainly audible, regardless of the quality of the
recording.

Oh, absolutely, but sometimes that's because of what the distortions do
to the artifacts in the original recording.

I like to use a particular track from Hair for listening to speaker systems...
something in the vocal chain on that track (2-4-0-0) is right on the edge
of clipping and the problem is much more audible on good speakers than bad
ones.

The irony being that it sounds 'worse' on 'good' speakers.

This is why domestic hi-fi tends to have little in common with studio monitors.
Horses for courses and all that.

Graham

 

[Eeyore]

Arny Krueger wrote:

"William Sommerwerck" <grizzledgeezer@comcast.net> wrote in message
news:qtGdndAVmYMhW1TbnZ2dnUVZ_hisnZ2d@comcast.com...
And somehow, magically, the golden-ear boys (is's almost always
boys) think that it matters that what they do to the signal that
comes *off* the CD makes so much difference that they can hear
the difference in the oxygen content of the interconnect wiring,
or 0.06 percent distortion when the producer added 30% of his
own, because he liked the effect.

What you say is intellectually logical, but it seems that post-recording
distortions can be plainly audible, regardless of the quality of the
recording.

30 years of experience with bias-controlled listening tests says that
*seems* and *is* can be two different things. Intellectual logic has this
interesting tendency to rule, once the comparison is based on just
listening.

When I reviewed, I made final judgements with my own live, undoctored
recordings.

The fun begins when you level-match, time-synch and eliminate other
non-audible cues.

I'd love to see you level match the sound field from different loudspeakers !

Graham

 

[Arny Krueger]

"Eeyore" <rabbitsfriendsandrelations@hotmail.com> wrote in message
news:46C9DDC1.681821F2@hotmail.com...

Mark wrote:

Eeyore wrote:
There was part of a thread a while back about how adding negative
feedback can
create higher order harmonic distortion products than exist open-loop
in an
amplifier stage.

This premise is NOT correct. Do not believe everything you read on
the Internet.

Feedback done correctly ADDS nothing. Perhaps what you are thinking
about is that feedback is generally more effective at reducing low
order distortion compared to reducing high order distortion. Feedback
(implemented correctly) does not INCREASE either form of distortion.
It reduces them both.

I know it decreases overall THD numbers. I'm not one of those nuts who's
anti-NFB
per se.

What is the case AIUI is that NFB can create 'new' (higher) harmonics
that don't
exist with the open-loop situation. It's down to the maths of how feedback
works.

The actual fraction of new higher harmonic is very low in practice.

Take a really crappy power amp that has a 1% nonlinearity.

If you put in a FS 1 KHz tone, you get out a nearly FS 1KHz tone, DC that is
46 dB down, and 2 KHz that is 46 dB down. Run the output back through again,
and you get a nearly FS 1 KHz tone, DC that is still about 46 dB down, a 2
KHz tone that is about 46 dB down, and a 3 KHz tone that is about 92 dB
down.

1. Higher harmonics *are* more audible, but the additional 46 dB down is
far, far less than enough to make up the difference.

2. If the amp has 0.1 % nonlinearity (still fairly crappy by modern
standards), the numbers are 66 dB down for the DC and second harmonic, and
132 dB down for the third harmonic.

3. If the amp has 0.01 % nonlinearity (very good modern standards), the
numbers are 86 dB down for the DC and second harmonic, and 172 dB down for
the third harmonic.

Most modern power amps will be someplace between (2) and (3). Most modern
op amp applications will be closer to (3), on the good side.

 

[Mark]

On Aug 20, 2:30 pm, Eeyore <rabbitsfriendsandrelati...@hotmail.com>
wrote:

Mark wrote:
Eeyore wrote:
There was part of a thread a while back about how adding negative feedback can
create higher order harmonic distortion products than exist open-loop in an
amplifier stage.

This premise is NOT correct. Do not believe everything you read on
the Internet.

Feedback done correctly ADDS nothing. Perhaps what you are thinking
about is that feedback is generally more effective at reducing low
order distortion compared to reducing high order distortion. Feedback
(implemented correctly) does not INCREASE either form of distortion.
It reduces them both.

I know it decreases overall THD numbers. I'm not one of those nuts who's anti-NFB
per se.

What is the case AIUI is that NFB can create 'new' (higher) harmonics that don't
exist with the open-loop situation. It's down to the maths of how feedback works.

Graham

And I am saying NFB CANNOT create new higher harmonics.

OK maybe what you are thinking aobut is this. If you take an
amplifier without feedback and overdrive it to clipping, it will
compress softly (2nd and 3rd order). If you take the same amplifier
and add NFB to it and now overdrive it to clippiong, it will remain
linear until it clips hard and this creates more high order harmoincs
comapred to the first case.
If you are talking about gutiar amp output stages where you are
overdriving into clipping intentionally, yeah feedback will make the
amp linear until it hard clips and lots of high order products are
created. An amp without feddback will overdrive with softer
compression and less higher harmonics.



But this is comparing apples and oranges. If you operate both below
clipping, the amplifer with neg feedback will have less overall
distortion and less high order distortion.


So are you talking about amps that are intentionally overdriven or
amps that are operated in their linear range?

Mark

 

[William Sommerwerck]

The source of this myth is the mistaken idea that negative feedback
regenerates the audio signal, and the nonlinearity of the amplifier
leads to higher order products of the regenerated harmonics and the
nonlinearity of the amp. This ignores the fact that the regenerated
signal is brought back in out-of-phase, and has the primary result
of reducing the high order harmonics.

I'm not sure that's right. My memory (which could be faulty) is that this
can be shown mathematically.

I'll ask around (I know a few people in high places) and see if I can get a
reference.

 

[William Sommerwerck]

What you say is intellectually logical, but it seems that post-recording
distortions can be plainly audible, regardless of the quality of the
recording.

30 years of experience with bias-controlled listening tests says that
*seems* and *is* can be two different things. Intellectual logic has this
interesting tendency to rule, once the comparison is based on just
listening.

When I reviewed, I made final judgements with my own live,
undoctored recordings.

The fun begins when you level-match, time-synch and eliminate other
non-audible cues.

Arny, I was talking in general terms. The distortion in recordings does not
automatically mask distortions further down the playback chain.

 

[Bob Myers]

"Mark" <makolber@yahoo.com> wrote in message
news:1187639180.495642.193740@i38g2000prf.googlegroups.com...

What is the case AIUI is that NFB can create 'new' (higher) harmonics
that don't
exist with the open-loop situation. It's down to the maths of how
feedback works.

Graham

And I am saying NFB CANNOT create new higher harmonics.

I'd like to see what Graham is using re the "maths of
how feedback works."

The complete result for a negative-feedback amplifier,
using the configuration typically shown for an
"inverting amp" application of an op-amp (but which
may be generalized to any amplifier) winds up
looking like this (assuming high enough input
impedance at the amplifier such that current into
that path is negligible):

Vout/Vin = Rf/[Rin(A-1)]

where A is the open-loop gain of the amp and Rf and
Rin are the feedback and input resistors, respectively.
Let A get big enough, and this simplifies to the more
common

Vout/Vin = -Rf/Rin

....but I sure don't see anything in the above which
would *necessarily* result in additional harmonics.
To be sure, in the practical case, the open-loop
gain of the amplifier is non-linear, but even then
you can clearly create an amplifier employing
negative feedback which does NOT "create new
harmonics" to an appreciably greater degree than its
open-loop cousin.

As a side note, it's really, REALLY hard to find any
practical amplifier design which doesn't involve
SOME negative feedback, somewhere - although it
may not be as easy to spot as in this case.

Bob M.

 

[Arny Krueger]

"Eeyore" <rabbitsfriendsandrelations@hotmail.com> wrote in message
news:46C9DFF6.60FE7BAC@hotmail.com...

Arny Krueger wrote:

"William Sommerwerck" <grizzledgeezer@comcast.net> wrote in message
news:qtGdndAVmYMhW1TbnZ2dnUVZ_hisnZ2d@comcast.com...
And somehow, magically, the golden-ear boys (is's almost always
boys) think that it matters that what they do to the signal that
comes *off* the CD makes so much difference that they can hear
the difference in the oxygen content of the interconnect wiring,
or 0.06 percent distortion when the producer added 30% of his
own, because he liked the effect.

What you say is intellectually logical, but it seems that
post-recording
distortions can be plainly audible, regardless of the quality of the
recording.

30 years of experience with bias-controlled listening tests says that
*seems* and *is* can be two different things. Intellectual logic has this
interesting tendency to rule, once the comparison is based on just
listening.

When I reviewed, I made final judgements with my own live, undoctored
recordings.

The fun begins when you level-match, time-synch and eliminate other
non-audible cues.

I'd love to see you level match the sound field from different
loudspeakers!

Of course we can't do that with anything like the precision that is readily
obtainable with amps, preamps, converters, digital players and recorders,
etc. OTOH, it can be done well enough to be revealatory to many.

 

[Arny Krueger]

"William Sommerwerck" <grizzledgeezer@comcast.net> wrote in message
news:6sOdnYd7_bqwa1TbnZ2dnUVZ_hWdnZ2d@comcast.com...

What you say is intellectually logical, but it seems that post-recording
distortions can be plainly audible, regardless of the quality of the
recording.

30 years of experience with bias-controlled listening tests says that
*seems* and *is* can be two different things. Intellectual logic has this
interesting tendency to rule, once the comparison is based on just
listening.

When I reviewed, I made final judgements with my own live,
undoctored recordings.

The fun begins when you level-match, time-synch and eliminate other
non-audible cues.

Arny, I was talking in general terms. The distortion in recordings does
not
automatically mask distortions further down the playback chain.

Since almost nothing in audio is automatic, that sounds a lot like an
excluded middle argument. ;-)

It is quite clear that the background noise in recordings is typically so
much greater than that in good equipment (other than microphones and rooms),
that it *does* mask the noise in much good equipment. True for good digital
recorders and players. True for many mic preamps, at least when used with
typical condenser mics.

The most common cause of audible distortion in audio gear is clipping due to
importune gain setting.

 

[Arny Krueger]

"William Sommerwerck" <grizzledgeezer@comcast.net> wrote in message
news:x72dnQmcwZ8FaFTbnZ2dnUVZ_vShnZ2d@comcast.com...

The source of this myth is the mistaken idea that negative feedback
regenerates the audio signal, and the nonlinearity of the amplifier
leads to higher order products of the regenerated harmonics and the
nonlinearity of the amp. This ignores the fact that the regenerated
signal is brought back in out-of-phase, and has the primary result
of reducing the high order harmonics.

I'm not sure that's right. My memory (which could be faulty) is that this
can be shown mathematically.

I have just laid out the math results in two other posts. I've confirmed it
with simulations.

 

[Arny Krueger]

"Mark" <makolber@yahoo.com> wrote in message
news:1187639180.495642.193740@i38g2000prf.googlegroups.com...

And I am saying NFB CANNOT create new higher harmonics.

But it can. However, if the NFB is working reasonably, the higher harmonics
are at vanishing low levels.

OK maybe what you are thinking aobut is this. If you take an
amplifier without feedback and overdrive it to clipping, it will
compress softly (2nd and 3rd order).

That avoiding NFB necessarily leads to soft clipping is a myth.

NFB does do a nice job of making such clipping as might happen have sharper
corners.

Tubes saturating can be a little soft. Transistors saturating or cutting off
give pretty sharp corners.

If you take the same amplifier
and add NFB to it and now overdrive it to clippiong, it will remain
linear until it clips hard and this creates more high order harmoincs
compared to the first case.

If the equipment is clipping, there are going to be plenty of higher
harmonics like 5,7 (presuming no P-P), even without NFB.

But this is comparing apples and oranges. If you operate both below
clipping, the amplifer with neg feedback will have less overall
distortion and less high order distortion.

Agreed.

 

[William Sommerwerck]

The most common cause of audible distortion in audio gear
is clipping due to importune gain setting.

Actually, clipping is not "in" the gear.

I do wish you'd glom onto a Crown K1. You really need to hear this
amplifier -- and run it through some blind tests.

 

[William Sommerwerck]

The complete result for a negative-feedback amplifier,
using the configuration typically shown for an
"inverting amp" application of an op-amp (but which
may be generalized to any amplifier) winds up
looking like this (assuming high enough input
impedance at the amplifier such that current into
that path is negligible):

Vout/Vin = Rf/[Rin(A-1)]

where A is the open-loop gain of the amp and Rf and
Rin are the feedback and input resistors, respectively.
Let A get big enough, and this simplifies to the more
common

Vout/Vin = -Rf/Rin

...but I sure don't see anything in the above which
would *necessarily* result in additional harmonics.

Because the mathematical model you're using doesn't include distortion!

To be sure, in the practical case, the open-loop
gain of the amplifier is non-linear, but even then
you can clearly create an amplifier employing
negative feedback which does NOT "create new
harmonics" to an appreciably greater degree than its
open-loop cousin.

How do you know that?

As a side note, it's really, REALLY hard to find any
practical amplifier design which doesn't involve
SOME negative feedback, somewhere - although it
may not be as easy to spot as in this case.

Many amplifiers include negative feedback, if only to stabilized local gain.

 

[Arny Krueger]

"William Sommerwerck" <grizzledgeezer@comcast.net> wrote in message
news:sqidnYix37lSnlfbnZ2dnUVZ_rWtnZ2d@comcast.com...

The most common cause of audible distortion in audio gear
is clipping due to importune gain setting.

Actually, clipping is not "in" the gear.

Please explain.

I do wish you'd glom onto a Crown K1. You really need to hear this
amplifier -- and run it through some blind tests.

Your'e externalizing again William. It is you who need to run your K1 nd a
QSC USA 400 or Dyna ST-120 through some level-matched bias-controlled
listening tests.

 

[Scott Dorsey]

Mark <makolber@yahoo.com> wrote:

And I am saying NFB CANNOT create new higher harmonics.

Sure it can. Just put something that creates harmonics in the feedback
path.

Negative feedback relies on the feedback path being linear and having
low group delay. If these aren't the case, bad things can happen.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."